Greek sea urchins see everything

Joe Mason: July 2011

Sea urchins are the bane of many Greek holidaymakers and stepping on one of the spiny creatures on the beach can be a real pain. Now scientist believe the sea urchin's body may act as a 'giant eye' that can detect objects in almost any direction. Researchers in Norway, Sweden and Germany have published a study in the journal PNAS National Academy of Sciences.

The study conducted at university marine biology departments concludes "the adult sea urchin can act like a giant eye and the shadow cast skeleton of the animal on the photosensitive cells of the body may give directional vision."

Sea urchins can be found on many Greek island beaches, especially on the stony stretches in sheltered coves loved by many holidaymakers.

Many animals have very complex eyes while others cope without any organs of vision whatsoever, but sea urchins, it appears, can see without specialised organs. They can see with their entire body, which somehow acts like a huge eye.

Previous studies have shown that sea urchins have a large number of genes associated with the development of the retina — those light-sensitive tissues found in the back of the human eye.

It was a puzzle why these genes should have survived in the sea urchin. The new study examined whether light sensitivity is localized at specific points in order to detect some photo-receptors.

It was a question of where the sea urchin exactly 'sees' things. Scientists found that the light receptors are not localized, as in normal eyes, but are scattered across both the top and base of the animal.

They found the 'vision' ability of the sea urchin is spread virtually throughout the whole body. The urchins have eyes everywhere, so they can see you coming from virtually any direction.